You are right about the motor set, it is normally 3 degrees back to get your pinion angle right. My 2cents worth is to look at the transmission mount before motor mounts and maybe that it would be easier to lower it to get the motor level to what you want. Nothing wrong with "too low" unless the oil pan is hitting cross member or sitting down lower than the cross member where it will take a shot from curbs/rocks/road debris.

I assume your mounts are aftermarket side mounts with rubber donuts that are bolted to the each side of the frame? The ones my truck had on it were cheap and sourced from the far east. They have since been discarded and will not be reused.

R-Bo

1959 Apache 1/2 ton Big Window, Short bed Fleetside (under reconstruction)
1966 GMC 305V6 in the family